If after using hddguru to low level format followed by Windows fat 32 full format the USB still wont flash...use PC to have a look at the image you have been trying to load:-
inside the zgemma folder inside there is another folder (I think its h1 on the early zgemma's) open it & you will see a few files...one will be called noforce right click & rename this file to force
now prep the USB once again, hddguru to low level format then a full not quick Windows format, next place the zgemma folder you just altered onto the USB & place in rear USB port of zgemma, power the box on via its white button on the back of the box then watch it flash the box, no need to press any buttons just power the box up & it should auto detect then flash.... IF you have followed the steps correctly & the box still wont flash then its not the box at fault it will be the USB pen drive thats duff....try another (small USB pen drives such as 2gb - 4gb are perfect for flashing, dont try using a huge drive like a 64gb pen drive as it will take hours to prepare)